1. Field of the Invention
The present invention relates to a wet friction material used in oil, particularly in an automatic transmission fluid (ATF), and to a phenol resin composition for the wet friction material used as a binder for the wet friction material.
2. Description of Background Art
Since wet friction materials for automatic transmissions for automatic vehicles are used in oil, the material must be thoroughly impregnated with oil. Such a wet friction material is therefore made from a porous material. A commonly used wet friction material is prepared by impregnating a paper body made of a fibrous material with a phenolic resin diluted in an organic solvent, followed by drying and heat-curing steps. As a phenolic resin, a resol-type phenol resin using an organic solvent such as an alcohol and ketone as a solvent is commonly used due to excellent heat resistance and excellent adhesion with the fibrous material. As a fibrous material, wood pulp, cotton lint, aramid fiber, and the like can be given. For preparing the paper body, fillers such as a friction-adjusting agent, solid lubricant, and the like are added as appropriate.
In recent years, downsizing and lightening of automatic transmissions are progressing aiming at a low fuel consumption in view of energy saving and environment protection. For this reason, miniaturization and a decrease in the number of wet friction materials used for the automatic transmissions are desired. Because the torque capacity is insufficient if conventional wet friction materials are used, an increase in the coefficient of friction is desired. If the coefficient of friction is increased, the materials generate a greater heat of friction. Therefore, improvement in durability of the wet friction materials is desired. The durability of wet friction materials is expressed by a permanent cycle number (life of the wet friction material) until the friction material exfoliates and the total thickness changes for the wet friction material itself. Therefore, three parameters, i.e. an increase in the coefficient of friction, an increase in the number of permanent cycles, and decrease of the total thickness change, are strongly desired for wet friction materials. Similarly, binders contributing to these three parameters are desired.
As a material for satisfying this requirement, Japanese Patent Application Laid-open No. 1996-259782 discloses a composite material consisting of a heat-curable resin and a metal oxide, a method of manufacturing such a composite material, and the like. The manufacturing method comprises hydrolyzing and poly-condensing a metal alkoxide containing a silicon alkoxide or its low condensate in a solution of a heat-curable resin containing a phenol resin, while removing the solvent or curing the resin, whereby a composite material with metal oxide particles having an average particle size of 0.01 to 5 xcexcm homogeneously dispersed in the heat-curable resin in the amount of 1 to 35 wt % in such a manner that no macro-phase separation may occur can be obtained. Among numerous applications of the composite material, the use as a friction material is mentioned. Japanese Patent Application Laid-open No. 2000-336177 discloses a wet friction material made by impregnating a paper substrate with a heat-curable resin. The patent application describes that this wet friction material exhibits an improved strength without decreasing porosity due to either a three-dimensional network structure of SiO2 or dispersed SiO2 particles in the heat-curable resin. A mixture of a hydrolysis solution of tetraalkoxysilane and a phenol resin is used as a binder for the wet friction material in the examples. Materials containing SiO2 in the amount of 24.5 to 26.7 wt % in the resins and the like are given as examples.
As conventional composite materials of heat-curable resin and metal oxide have improved sliding characteristics such as particularly controlled friction characteristics, a high limit PV value, and low wearing characteristics, but do not necessarily possess a high coefficient of friction as a binder for wet friction materials, further improvement was desired. In addition, the content of SiO2 in the SiO2-containing heat-curable resin is no more than 30 wt %, which is not necessarily sufficient for ensuring a high coefficient of friction, an increased number of permanent cycles, and a decrease in the total thickness change.
Accordingly, an object of the present invention is to provide a phenol resin composition for wet friction material which can satisfy the requirements for a high coefficient of friction, a decrease in the total thickness change, and an increased number of permanent cycles until the friction material is peeled off at the same time, as well as a wet friction material. Another object of the present invention is to improve the storage stability of the phenol resin composition and improve usability of the phenol resin composition as a binder for wet friction material.
Specifically, the present invention provides a phenol resin composition for wet friction material comprising a curable resin composition obtained by mixing a resol-type phenol resin and a hydrolysis solution of an alkoxysilane or a condensate thereof, wherein the ratio (R/S) by weight of the nonvolatile components of the resol-type phenol resin (R) and the nonvolatile components of the hydrolysis solution of the alkoxysilane or the condensate thereof (S), obtained after heat-treatment at 135xc2x0 C. for one hour, is in the range of 64/36 to 10/90.
The present invention also provides a wet friction material in which the above phenol resin composition for wet friction material is used as a binder.
The present invention further provides a wet friction material product with the above wet friction material attached to one or both sides of a core material using an adhesive.
The present invention will be described in detail below. The phenol resin composition for wet friction material of the present invention is a liquid resin composition which is a binder precursor for a wet friction material. Although not specifically limited, the resol-type phenol resin used in the present invention can be prepared by mixing a phenol compound (P) and an aldehyde compound (A) at a molar ratio of A/P=0.8 to 1.4, preferably 0.9 to 1.3, reacting the mixture in the presence of an alkali catalyst, removing water by distillation under reduced pressure to a water content of 10 wt % or less, and dissolving the resulting product in alcohol. The product with a molar ratio of more than 1.4 has such a large cross-linking density that when the product is used in a wet friction material, the total thickness change may unacceptably increase. If the molar ratio is less than 0.8, phenol compounds may remain unreacted, resulting in a decreased yield of the resin. The number average molecular weight of the resol-type phenol resin is preferably from 150 to 400, and particularly preferably from 180 to 350. If the number average molecular weight is less than 150, monomers may remain unreacted, resulting in a decreased yield of the resin. If the number average molecular weight is more than 400, the fibrous material is impregnated with the resin only with difficulty and the resin exhibits impaired adhesion with the fibrous material, resulting in a composite material in which the number of permanent cycles under high surface pressure can be improved only with difficulty when used as a wet friction material. Here, the number average molecular weight is a polystyrene-reduced number average molecular weight measured by using HPLC. Because the resol-type phenol resin of the present invention can be obtained by the above method, the resin may be used in the form containing an alcohol and a small amount of water when preparing an impregnation solution by mixing it with the hydrolysis solution of an alkoxysilane or a condensate thereof, described later in this specification.
There are no specific limitations to the phenol compound (P) used in the present invention. Phenol, cresol, xylenol, m-cresol, m-ethyl phenol, resorcin, catechol, hydroquinone, bisphenol A, and the like can be given as examples. These compounds may be used either individually or in combination of two or more. Although there are no specific limitations to aldehyde compounds (A) used in the present invention, formaline, paraformaldehyde, trioxane, acetoaldehyde, and benzaldehyde can be given as examples. These aldehyde compounds may be used either individually or in combination of two or more.
Although there are no specific limitations to alkoxysilanes used in the present invention, an alkoxysilane having two or more hydrolyzable alkoxy groups in the molecule can be used. Specific examples are tetramethoxysilane, tetra ethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, phenyl trimethoxysilane, phenyltriethoxysilane, diphenyl dimethoxysilane, and diphenyldiethoxysilane. Although there are no specific limitations to the alkoxysilane condensates, a condensate with an average degree of polymerization of about 4 to 5 in the form of a liquid with excellent storage stability is preferably used. Specifically, condensates slightly condensed to the extent that the liquid storage stability is sufficiently maintained are desirable. Specific examples include condensates of tetraethoxysilane (xe2x80x9cEthylsilicate 40xe2x80x9d, xe2x80x9cEthylsilicate 48xe2x80x9d manufactured by Colcoat Co., Ltd.), a condensate of tetramethoxysilane (xe2x80x9cMethylsilicate 51xe2x80x9d manufactured by Colcoat Co., Ltd.), and the like. These alkoxysilane condensates may be used either individually or in combination of two or more.
Although there are no specific limitations to the hydrolysis solution of an alkoxysilane or a condensate thereof used in the present invention, such a hydrolysis solution is typically obtained by charging a reaction vessel such as a flask equipped with a stirrer with an alkoxysilane or a condensate thereof, an alcohol, water, and an acid and reacting the mixture for 1-5 hours at room temperature to 50xc2x0 C. The alcohol used herein includes, but is not limited to, methanol, ethanol, isopropanol, and butanol. These alcohols may be used either individually or in combination of two or more. The alcohol must be added in an amount sufficient to obtain a clear, homogeneous solution. The amount of water added is preferably 80 to 250% of the theoretical amount of water (100%) necessary for ultimately gelling all the hydrolyzable alkoxysilane groups after hydrolysis and condensation. If the amount of water is too small, a sufficient hydrolysis reaction will not occur; if too large, storage stability of the hydrolysis solution is decreased. Water may be added either at one time or in several portions. Although there are no specific limitations to the acid, a diluted aqueous solution of hydrochloric acid, sulfuric acid, or nitric acid with a concentration of about 1 N is used. These acids may be used either individually or in combination of two or more. The acid is used in an amount of about 30 g (as a 1 N aqueous solution) or less for 1 kg of alkoxysilane or its condensate. A mixture of alkoxysilane or its condensate may be used as a material for hydrolysis.
The resin composition of the present invention is a fluid mixture of a resol-type phenol resin and a hydrolysis solution of an alkoxysilane or a condensate thereof. The resol-type phenol resin and the hydrolysis solution of an alkoxysilane or a condensate thereof are mixed at a proportion so that the ratio by weight of the nonvolatile components of the resol-type phenol resin (R) and the nonvolatile components of the hydrolysis solution of the alkoxysilane or the condensate thereof (S), obtained after heat-treatment at 135xc2x0 C. for one hour, may be in the range of R/S=64/36 to 10/90, and preferably 60/40 to 10/90. Specifically, the resol-type phenol resin containing an alcohol and the like and the hydrolysis solution of an alkoxysilane or its condensate are respectively treated with heat at 135xc2x0 C. for one hour to determine their respective nonvolatile components. The proportion of the resol-type phenol resin and the hydrolysis solution of alkoxysilane or its condensate is determined so that the ratio R/S is in the above range. If the proportion of the nonvolatile components originating from the hydrolysis solution of alkoxysilanes is less than 36 wt % in all nonvolatile components, the wet friction material may not have a sufficiently improved number of permanent cycles; if more than 90 wt %, the total thickness change of the wet friction material tends to become large.
Although there are no specific limitations to the method of mixing the resol-type phenol resin and the hydrolysis solution of an alkoxysilane or a condensate thereof, a typical method is dripping a homogeneous hydrolysis solution of an alkoxysilane or its condensate into a prescribed amount of solution of the resol-type phenol resin in an alcohol, while the latter is stirred, until a prescribed amount of the hydrolysis solution is added, affording a homogeneous solution. The amount of nonvolatile components in the mixed solution may be adjusted by adding an alcohol during preparation of the mixture so that a prescribed amount of resin composition may be attached to the paper body of wet friction material. The content of the nonvolatile components in the liquid resin composition of the present invention is preferably 10 to 45 wt % to ensure superior storage stability and to cause the paper body to be homogeneously impregnated with the resin composition.
The phenol resin composition of the present invention for wet friction material is preferably adjusted to pH 2.0 to 4.5, and more preferably to 2.5 to 3.5, by the addition of an acid. If the pH is outside the above range, the storage stability of the liquid phenol resin composition is so impaired that the resin composition may not be industrially used. The acid may be added either when mixing the resol-type phenol resin and the hydrolysis solution of an alkoxysilane or its condensate or may be previously added to the phenol resin. The acids which can be used include, but are not limited to, hydrochloric acid, sulfuric acid, nitric acid, p-toluene sulfonic acid, formic acid, acetic acid, and lactic acid. The pH of the phenol resin in the present invention indicates the pH value determined for a 1:1 mixture (by weight) of the phenol resin and purified water at 25xc2x0 C.
A silane coupling agent or a hydrolyzate solution thereof is preferably added to the resin composition of the present invention to improve adhesion of the substrate and binder in the ultimately obtained wet friction material and to improve adhesion of the phenol resin and silicon oxide in the binder. Although there are no specific limitations to the type of silane coupling agent, at least one of the silane coupling agents of the following formula (1) or its hydrolyzate solution is used.
Yxe2x80x94(CH2)nxe2x80x94SiX2Zxe2x80x83xe2x80x83(1) 
wherein X is a methoxy group or an ethoxy group, Y represents an amino group, ureido group, mercapto group, aminoethylamino group, 2,3-epoxypropoxyl group, 3,4-epoxycyclohexyl group, or anilino group, Z is a methoxy group, ethoxy group, or methyl group, and n indicates an integer from 1 to 3.
Specific examples are xcex3-glycidoxypropyltrimethoxy silane (A-187, manufactured by Nippon Unicar Co., Ltd.), xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (A-186, manufactured by Nippon Unicar Co., Ltd.), xcex3-aminopropyl triethoxysilane aqueous solution (A-1106, manufactured by Nippon Unicar Co., Ltd.), and the like. The amount of the silane coupling agent is preferably 0.1 to 10 wt % of the phenol resin composition.
The wet friction material of the present invention contains the nonvolatile components originating from the above liquid phenol resin composition as a binder, and usually comprises a fibrous material, a filler, a friction adjustment agent, and the binder. Specifically, the wet friction material is prepared by preparing a mixture of fiber such as natural pulp fiber or aramid fiber, filler such as diatomaceous earth, and a friction adjustment agent, dispersing the mixture in water to produce a slurry, and milling the slurry into paper, drying the paper to obtain a paper body, impregnating the paper body with the liquid phenol resin composition, and drying and curing the resin composition. The wet friction material thus obtained may be fabricated into a desired form such as a circular ring, as required. The binder in the wet friction material contains the resol-type phenol resin and the hydrolysis solution of an alkoxysilane or its condensate at a nonvolatile component ratio by weight of R/S=64/36 to 10/90. The nonvolatile components are components that do not volatilize when heated at 135xc2x0 C. for one hour. The impregnation amount of the binder in the paper body is preferably 20 to 80 parts by weight for 100 parts by weight of the paper body on a dry basis.
The wet friction material thus obtained has a high coefficient of friction, exhibits only a minimal change in the total thickness, and exhibits an improved number of permanent cycles before delamination. The reason is as follows. Through the heat treatment in the process for manufacturing the wet friction material, the alkoxysilane hydrolysis solution is cured into a product having a siloxane bond with high flexibility and bending properties. Due to the nonvolatile component ratio in the range of 36 to 90 wt % in all nonvolatile components, the cured product increases the contact area of the wet friction material and the object to be adhered therewith, which is thought to give rise to an increased coefficient of friction. In addition, because the bonding energy of Sixe2x80x94O is significantly larger than the boding energy of Cxe2x80x94C that forms the main skeleton of organic resins such as a phenol resin, introduction of siloxane bonds is thought to result in an increase in the number of permanent cycles before delamination of the wet friction material. On the other hand, the total thickness change of the wet friction material increases due to the high flexibility and bending properties of the siloxane bond. The total thickness change of the wet friction material can be maintained at the same level as that of the resol-type phenol resin by adding 64 to 10 wt % of the resol-type phenol resin which becomes cured by heating. The storage stability of the mixed solution of the alkoxysilane hydrolysis solution and the resol-type phenol resin is greatly affected by the pH of the solution. Adjusting the pH in an optimal range by the addition of an acid can ensure stable storage over a long period of time.
The wet friction material product of the present invention is made by causing the above wet friction material to adhere to one or both sides of a core material using an adhesive. Specific examples include a friction board consisting of a doughnut-shaped metallic core plate provided with a number of teeth around the inner or outer circumference and a circular ring made from the wet friction material attached to the both sides of the core plate, a break band made from a circular arc-shaped metallic thin plate with both ends being clamped with brackets and the wet friction material adhering in the inner side of the metallic thin plate, a lock-up clutch made from a core plate and the wet friction material adhering around the front cover side of the outer circumference of the core plate, and the like. As a means for causing the wet friction material to adhere to the core, a method of causing the same shape of the wet friction material as the core to adhere, a method of causing a circular ring of the wet friction material to adhere a disc-shaped core, a method of causing many pieces of the wet friction material in the shape of a square, sector, or circle, or non-specific shape to adhere at regular intervals, and the like can be given. Known adhesives can be used to cause the wet friction material to adhere to the cores.